Canopy transpiration in two Japanese cypress forests with contrasting structures

Canopy transpiration (E_C) varies among forest stands with different structures. To evaluate different E_C for Japanese cypress we observed E_C for 5 months, by use of the sap flow technique, in two adjacent stands of different ages with contrasting structures. Mean diameter at breast height was 13.5 and 44.6 cm and stem density was 2100 and 350 trees ha⁻¹ for the Sakuta and Hiwada plots, respectively. Mean E_C measured was 1.3 mm day⁻¹ for the Sakuta plot and 0.7 mm day⁻¹ for the Hiwada plot (or 53 % of that for the Sakuta plot). This difference between E_C was caused by differences between both mean stand sap flux density (J_S) and stand sapwood area (A_{S__stand}): J_S for the Hiwada plot was 69 % of that for the Sakuta plot and A_{S_stand} for the Hiwada plot was 75 % of that for the Sakuta plot. The difference between J_S was primarily caused by different reference J_S for given meteorological conditions, not by the different response of J_S to meteorological conditions. Previous studies of coniferous plantation forests in Japan reported that differences between E_C among stands with different structures were mainly caused by different A_{S_stand}. This study revealed this is not always true, and that differences between J_S should also be considered when predicting differences between E_C among stands with different structures.